Study: Warming Causing Decline in Global Crop Production

16 March 2007

The solid line shows the mean estimate of climate effect on yield trends from 1981 to the year shown on the x axis. Dotted lines indicate 95% confidence interval. Click to enlarge.

Over a span of two decades, warming temperatures have caused annual losses of roughly $5 billion for major food crops, according to a new study by researchers at the Carnegie Institution and Lawrence Livermore National Laboratory.

From 1981-2002, warming reduced the combined production of wheat, corn, and barley by 40 million metric tons per year, according to the analysis published today in the online journal Environmental Research Letters.

The results suggest that recent climate trends, attributable to human activity, have had a discernible negative impact on global production of several major crops. The impact of warming was likely offset to some extent by fertilization effects of increased CO2 levels, although the magnitude of these effects are uncertain and the subject of much debate.

If each additional ppm of CO2 results in ~ 0.1% yield increase for C3 crops (a yield increase of 17% for a concentration increase from the current 380 ppm to the frequently studied 550 ppm), then the ~ 35 ppm increase since 1981 corresponds to a roughly 3.5% yield increase, about the same as the 3% decrease in wheat yield due to climate trends over this period.

Thus, the effects of CO2 and climate trends have likely largely cancelled each other over the past two decades, with a small net effect on yields. This conclusion, while tempered by the substantial uncertainty in yield response to CO2, challenges model assessments that suggest global CO2 benefits will exceed temperature related losses up to ~ 2° warming.

The study is the first to estimate how much global food production has already been affected by climate change. Christopher Field and David Lobell compared yield figures from the Food and Agriculture Organization with average temperatures and precipitation in the major growing regions.

They found that, on average, global yields for several of the crops responded negatively to warmer temperatures, with yields dropping by about 3-5% for every 1° F increase. Average global temperatures increased by about 0.7° F during the study period, with even larger changes in several regions.

Though the impacts are relatively small compared to the technological yield gains over the same period, the results demonstrate that negative impacts are already occurring.

—David Lobell

The researchers focused on the six most widely grown crops in the world: wheat, rice, maize (corn), soybeans, barley and sorghum—a genus of about 30 species of grass raised for grain. These crops occupy more than 40% of the world’s cropland, and account for at least 55% of non-meat calories consumed by humans. They also contribute more than 70% of the world’s animal feed.

The main value of this study, the authors said, was that it demonstrates a clear and simple correlation between temperature increases and crop yields at the global scale.

The Carnegie Institution of Washington has been a private nonprofit research organization since 1902. It has six research departments: the Geophysical Laboratory and the Department of Terrestrial Magnetism, both located in Washington, DC; The Observatories, in Pasadena, California, and Chile; the Department of Plant Biology and the Department of Global Ecology, in Stanford, California; and the Department of Embryology, in Baltimore, Maryland.

From the abstract of the paper, emphasis added: "Despite the complexity of global food supply, here we show that simple measures of growing season temperatures and precipitation—spatial averages based on the locations of each crop—explain [b]~30% or more of year-to-year variations[/b] in global average yields for the world's six most widely grown crops."

Isn't this basically the same as saying that at least 65% (because surely they'd use the highest rounded number they could; if they could justifiably have said "35% or more" they would have) of the year-to-year variations in crop yield is due to factors [i]not[/i] related to climatological measures of temperature and precipitation?

I am not a statistician, but that seems to me like more than enough space for a reasonable alternate explanation of a 3-5% yield drop.

The loss of 40 million tons of grain is nothing to dismiss lightly, but the first rule of statistical analysis is that correlation does not imply causation.

Stephen, if you question the science of global warming too much, you may find yourself sacrificed on their altar of narrow-mindedness. They are currently going through the same thing the Catholic church went through in the Middle Ages, so let them figure it out before you get sucked in to the intolerance for other opinions in their new climate change religion. Remember, it wasn't so long ago that they were worried about global cooling, not warming. I guess the Prophet Al Gore is occasionallly allowed to change his mind without the followers questioning it. Have you seen his house? Jerry Fallwell should take lessons from him.........

"Global agricultural production potential is likely to increase with increases in global average temperature up to about 3 Celsius (5.4 Fahrenheit), but above this it is very likely to decrease," the draft said.

From FAO data shown in gramene.org, world maize production increased from 450 (1981) to 600 (2002) Mt, a 33% increase. In the FAOSTAT web site, world cereal production increased from 1,573 Gt (1979) to 2,086 Gt (2003). Data from other sources show a similar trend.

The Carnegie study specifically claims that the yield per hectare has decreased. This again is false. From gramene.org, "World maize production has increased almost 300% (see Figure 3), while the area harvested has increased only 33% (see Figure 4), indicating a large improvement in yield per hectare."

I got these numbers from a 10 min search on Google. Shame on the authors of this "study" and shame on those who believed them.

"We assumed that farmers have not yet adapted to climate change—for example, by selecting new crop varieties to deal with climate change. If they have been adapting—something that is very difficult to measure—then the effects of warming may have been lower,” explained Lobell.

All right, they didn't take account of people growing wheat, where previously it was too cold and that kind of thing,

even so, the decrease of 3% from warmer temperatures is compensated by a 3.5 increase from CO2 fertilisation.

That means, global warming to date has increased food production, not decreased it, and I believe their numbers are on the pessimistic side of what's credible.

Ah well, sensationalist, alarmist headlines do at least get you some attention ...

I can't beleive a bunch of talking heads are so sure they are right over people who study this for a living, publishing in peer reviewed journals, really, just because they don't like the conclusions. Why not make a bunch of ad hoc, uninformed hypotheses as to why they have UNDERESTIMATED losses due to GHGs? Perhaps because there is a little bias going on here? You can whine and winge and cite opiniated web sites as much as you like but until you publish your opinions based on evidence in a peer reveiwed scientific journal, I for one, won't take much more notice of you.

I don't care one way or another if this thread wants to get back into the global warming debate again. I only wanted to point out that recent droughts has significantly affected wheat production world wide.

I realize that some wants to only focus the debate on C02 and temperature readings, but I believe for crop productions, it's less about specifics, but rather about the changes for specific regions. For those who believe in the science of climate change, well, I think the lower production for wheat is a sign. For those who don't, then I'm sure wheat production will rebound eventually, no sweat.

Why should we accept this without question? My problem with it is that it seems to contradict the UN study, which expects a greener world and higher crop production through about 2050. Which group of scientists should I believe?

The analysis itself indicates that the effects of climate change are a mixed bag of increased yields due to CO2 fertilization and negative effects due to temperature increases.

For instance, manny misunderstands (at least I hope that's the reason) what the authors are concluding. They are not stating total production has decreased due to climate change. They are saying there are losses going on such that if climate change didn't exist yields would be greater than they currently are.

Read it again:

"Though the impacts are relatively small compared to the technological yield gains over the same period, the results demonstrate that negative impacts are already occurring."

Cervus, I don't really see the conflict going on here. Your cited web quotes are a prediction which concedes uncertainty. This study is a measurement of what is going on now.

Heiko, while the study does conclude with a slight net positive effect, why do you go on to state you believe these people are pessemists? What evidence of this do you have?

ps I realize that my first post was a bit of a rant. What irks me is that a lot of work goes into scientific publications by people who devote their lives to studying these problems. The work has to also go through peer review which means that other scientists in the field try to find fault and suggest or demand modification or outright reject it.

It seems like an all too familiar routine here that when a scientific study showing some negative effect from climate change is posted, it is instantly criticized using what ever silly arguments average Joe can come up with. I'm not saying that critisism or arguement is not desired here to some degree, afterall this is a blog site. But, as I mentioned, we never see anyone argue that its actually worse than what ever study indicates. Why is this?

Manny you are not taking into account the context of the quote. Even within this press statement it then goes on to say that the loss effects are offset by gains so how could they be trying to say total production is lower? What they are saying is that there is negative contribution due to climate change. Due to technology, increased carbon uptake etc this is more than compensated for such that total production has increased, as your cited data indicates.

they say there are losses compared to a world in which CO2 is higher but temperature isn't, and assuming no adaptation to higher CO2 or higher temperature.

They aren't saying there are absolute losses, though the headline jolly well makes it sound like that.

Marcus,

look at the headline for starters, doesn't that make it sound worse than what they've actually found?

The "study" Cervus cites is actually the upcoming IPCC assessment report, or rather a press story citing a draft, because the latest IPCC report isn't actually out yet.

The IPCC assessment report is not a study, it's a literature review, ie a condensed summary of all the important studies on the subject done by a large panel of scientists within the UN system of agencies.

It's only a sentence from a draft admittedly that Cervus is citing, but I don't expect the final version to say anything different.

I have to admit that some of the fault lies in these short and sometimes slightly misleading press excerpts. Tom its 40 million metric tons per year isn't it? But your interpretation is correct. Without climate change there would be 40 more million tons produced each year, still assuming increased CO2 however. In fact it seems that without CO2 increases and cilmate change together we would be at about the same as we are now. So little net change due to the whole CO2 scenario.

Heiko, you still haven't answered as why you think this study has a pessimistic bias. Granted the press headline is not very helpful but look at the title of the actual paper. “Global scale climate–crop yield relationships and the impacts of recent warming" Doesn't sound very alarmist to me.

you are right, there's no conflict. Having read the previous IPCC report, and this summary sentence for the latest version, I still believe that this particular study is relatively pessimistic, but it mightn't be by much, and I might be wrong and it's actually pretty middle of the road, especially when only considering studies neglecting adaptation.

The us is resilent because we use gm crops that are resilient to the climate changes. We also tend to buy alot of food from whereever it grows and can cntinue doing so as it grows elsewhere in 100 years.

Finaly the us is much more likely to embrace fake food aka phood and extreme GM foods as well as other foods that other people wont turn to.

Have any of you guys ever been a farmer, or planted crops? You are citing equations, and studies and charts like its some sort of cut and dried report. Farming is not at all like watching the Dow jones average. Cant just enter a bunch of numbers into a laptop and let it compute what should come out. Too many variables. I have planted corn in the past, and know all too well the effect of a heat wave on a crop on a certain period of its growth, or the effect of a rain on a crop, one week too late. Key words being "too late". Would be similar to giving man dying from dehydration a glass of water one week too late.

Farming is AT BEST,a crapshoot. Saying that as the world is warming, its increasing the available cropland available for use is absurd. A farmer in Texas is not going to be happy that new land is available for him to farm in Canada, because he is not going to go to Canada. He owns, or leases his lands in Texas. Record heat and droughts hurt him where he is, and doesnt open opportunities for him elsewhere. Just not that simple!

I remain a little skeptible about us being totally responsible for global warming. But since it currently appears to be, the main worry should be our food supplies. If our already precarious food supply is threatened by global warming, we should stop all efforts into trying to grow fuel for our vehicles, and instead grow food for our bellies. Where have our priorities gone?